1. Field of the Invention
The present invention relates to a transmission for a hybrid electric vehicle, and more particularly, to an electrically variable transmission (EVT) that implements a continuous transmission gear ratio, using one or more planetary gear sets and motors.
2. Description of Related Art
The EVTs in the related art can be classified into a single mode type and a multimode type. The single mode type, as shown in FIG. 1, typically uses a planetary gear set 500 that has a configuration in which input from an engine is supplied to a carrier C and output is achieved through a ring gear R, and a first motor generator MG1 is connected to a sun gear S and a second motor generator MG2 is connected to ring gear R.
The relationship of torque in the EVT can be briefly shown, as shown in FIG. 2, that is, torque from first motor generator MG1 and the output shaft is applied from both sides, with respect to torque supplied from the engine to carrier C, such that they keep balance.
Therefore, first motor generator MG1 should keep supplying appropriate reaction force to transmit the torque of the engine well to the output shaft, such that the torque of the output shaft is necessarily smaller than the torque of the engine. Further, the amount of power transmitted from the engine along an electric path which is made through first motor generator MG1 and second motor generator MG2 considerably increase in an acceleration section of the vehicle, such that efficiency decreases and motor generators having large capacity are required.
Although the multimode types of EVTs have been proposed to overcome the defects described above, most of the EVTs are provided with an ENGINE ONLY mode for starting a vehicle due to various limits for the characteristics of batteries and motor generators.
That is, in wide open throttle (WOT) start in which a driver rapidly starts a hybrid electric vehicle by maximally pressing down the acceleration pedal, it is difficult to ensure sufficient output shaft torque due to limits in the batteries and motor generators in the multimode types of EVTs of the related art, such that the specific ENGINE ONLY mode is provided for the rapid acceleration start only by the engine, such that the user's request is satisfied.
Further, as described above, in order to overcome the practical limits in the batteries and motor generators, various modes including the ENGINE ONLY start mode are provided and the number of clutches used to switch the modes is increased; however, as a plurality of clutches is provided, transmission efficiency of the EVT decreases, and new problems, such as complicity in the control of switching the modes and shock in switching the modes, occur.
Further, the shock due to switching the modes is generated and acceleration response of the vehicle decreases when the modes are frequently switched in the WOT start. In particular, it is impossible to continuously increase the number of revolution of the engine in the WOT start in the multimode type of EVT of the related art and it is required to reduce the number of revolution of the engine at a predetermined level in order to switch the modes, such that control is difficult, transmission shock is easily generated, and the acceleration response decreases.
For example, the EVT shown in FIG. 3 is a 2-mode EVT having a fixed gear ratio and can be illustrated by the lever diagram of FIG. 4. In the EVT, a vehicle is started at the first stage with a fixed gear ratio which is implemented by engaging only a firsts clutch CL1 with a fourth clutch CL4 in the WOT start and a first planetary gear set PG1 and a second planetary gear set PG2 become integral by the engagement of fourth clutch CL4 and rotate with the same number of revolution as the engine connected to an input shaft. Further, a third ring gear R3 of a third planetary gear set PG3 is fixed by first clutch CL1, such that the power of the engine that is inputted to a third sun gear S3 of third planetary gear set PG3 is reduced and transmitted the an output shaft through a third carrier C3.
As the vehicle speed increases, the engine is supposed to operate close to the maximum power point such that the maximum acceleration force is acquired, but as the vehicle speed increases, the operation point of the engine exceeds the maximum number of revolution.
In this state, the power of the engine is supposed to directly supplied from second carrier C2 to third carrier C3 by engaging a second clutch CL2, but in this state, there is necessarily a large difference between the rotation speeds of second carrier C2 and third carrier C3, such that it is required to synchronize the rotation speeds of second carrier C2 and third carrier C3 by reducing the speed of the engine. As a result, it becomes worse to control the EVT and transmission shock is likely to be generated, and the acceleration response decreases with the decrease in rotation speed of the engine.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.